The present invention pertains to color-balancing of photographic printing paper.
Although the present invention has application in various photographic processes, it is particularly useful with processes wherein a positive print is produced from a positive color transparency, using materials known as integral tripacks which have three superimposed light-sensitive emulsion layers. Those skilled in the art will appreciate that it is rarely, if ever, possible to produce a print with accurate color rendition without adjusting for color balance variations which are caused by color-mix changes in lighting conditions and changes in the characteristics of films and printing papers. Positive to positive processes provide substantial latitude in making color correction in printing the transparency by selective variation of red, green, and blue-light exposures.
There are two principal techniques for making such exposure variations in present use. In tricolor additive printing, three separate exposures of the transparency to the printing paper are made respectively through red, green, and blue separation filters, and the three exposures are varied to control color correction. Thus, the printing paper is exposed to varying amounts of red, green, and blue light (hereinafter referred to as the primary colors of light). In white-light subtractive printing, a single exposure of the positive to the printing paper is made through a combination of colored filters, wherein filter densities and colors are varied to control color correction, thereby selectively filtering out varying amounts of the primary colors of light.
In either case, color control is achieved by selectively varying the exposure of the primary colors of light to the printing paper, the exposure being basically the product of light intensity and duration. Such color light variations produce corresponding variations in the production of appropriately colored dyes by the three emulsion layers, red producing red, green producing green, and blue producing blue.
Most prior art color balancing techniques employ visual color judgments, using test prints. Such tests may be of typical subjects, in which case, in the white-light subtractive technique, the use of color filters for viewing a test print to make a subjective judgment as to filtering changes for making subsequent prints is in widespread use. Another technique is to use a matrix of cyan, magenta, and yellow dyes in steps equivalent to some multiple of "color-correcting" units (i.e. 5 cc or 10 cc) to produce a trial or test print by printing through the matrix and then making a visual judgment of the processed print relative to some standard such as a standard gray card. This latter technique suffers from two major defects. First, there is no precise correlation between the indicated degree of correction and the units of color correction found in enlarger colorheads of varying manufacture, to which must be added the crudity of the indicated corrections since 5 cc units is a very coarse measure of correction. The second and most incapacitating defect is that it is virtually impossible to test for three independent variables (red, green, and blue) on a two-dimensional medium.
Less subjective techniques involving the use of color analyzers to detect changes in color balance from negative to negative, or positive to positive, have been in use. Proper use of such techniques requires information as to the color balance of the printing paper being used. Due to changes in storage conditions and age, no two boxes of color printing paper have precisely the same color balance. This impediment has been removed completely by U.S. Pat. Nos. 4,166,701 and 4,257,702 so far as color printing from color negatives to color positive prints is concerned. Because of certain difficulties presented by the design of color analyzers, it is neither convenient nor possible to extend the techniques described in the afementioned patents to the control of color balance of the available materials when printing by positive to positive one-step processes. Some other means of equal accuracy for adjusting the printing to the varying color balance of different lots and different boxes of positive to positive printing paper had to be found.
The eye, quite correctly, has been described as the most discriminating instrument for detecting color differences when two samples are presented to it side by side. Sadly, it lacks the equally important capacity for precisely judging just how much difference there is between side by side samples. Some other technique has to be sought by which color differences may be quantitated. The most simple and accurate technique in color printing by which difference may be measured is the precise measurement of the exposure difference which produces a definite color change. The present invention combines the color comparing faculty of the eye with the precision of photosensitometry to control the color balance of positive to positive color printing so that the results rival those obtained by use of the techniques of the named patents when printing negative to positive color. The present invention, also, circumvents the difficulty posed by three variables in a two dimensional system by holding one color (for example, red) constant while varying the other two continuously against each other and against the fixed color, thus completely doing away with a basic fault of existing matrix-dye systems of color balancing. What results is an X-Y axis system of color balance, which in certain applications can give color analyzer program corrections as fine as 0.0025 log exposure units.